MINERALOGY of the MERCURIAN SURFACE. Kathleen E. Vander Kaaden1,2, Francis M. Mccubbin1,2, Larry R. Nittler3, Patrick N. Peplowski4, Shoshana Z

47th Lunar and Planetary Science Conference (2016) 1476.pdf

MINERALOGY OF THE MERCURIAN SURFACE. Kathleen E. Vander Kaaden1,2, Francis M. McCubbin1,2, Larry R. Nittler3, Patrick N. Peplowski4, Shoshana Z. Weider3, Larry R. Evans5, Elizabeth A. Frank3, Timothy McCoy6. 1Institute of Meteoritics, Department of Earth and Planetary Sciences, 1 University of New Mexico, MSC03- 2050, Albuquerque, NM 87131. 2NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058. 3Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015. 4The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723. 5Computer Science Corporation, Lanham- Seabrook, MD 20706. 6Smithsonian Institution, Washington, DC 20560, ([email protected]).

Introduction: The MErcury Surface, Space have used modified average compositions for these ENvironment, GEochemistry, and Ranging geochemical regions defined in [17] to compute the (MESSENGER) spacecraft orbited Mercury for four normative mineralogy of the mercurian surface. For years until April 2015, revealing its structure, chemical each composition, two normative compositions were makeup, and compositional diversity. Data from the calculated: 1) using only the measured Cr, Mn, and Ti mission have confirmed that Mercury is a compositional from XRS and 2) using the XRS detection limits of Cr, end-member among the terrestrial planets [1]. The X- Mn, and Ti reported in [3] as upper limit values for these Ray Spectrometer (XRS) and Gamma-Ray three elements. Due to the high amount of sulfur in Spectrometer (GRS) on board MESSENGER provided mercurian compositions, the CIPW normative the first detailed geochemical analyses of Mercury’s mineralogy calculations could not be conducted in the surface [e.g., 2–5]. These instruments have been used in conventional manner. Instead, we first calculated the conjunction with the Neutron Spectrometer and the sulfides that would be present in each composition using Mercury Dual Imaging System to classify numerous partition coefficients from [16], which were determined geological and geochemical features on the surface of for Mercury-relevant compositions and fO2 conditions. Mercury that were previously unknown. Furthermore, These data indicate that S bonds with Fe, Cr, Ti, Mn, the data have revealed several surprising characteristics Mg, and Ca, respectively, listed in descending order of about Mercury’s surface, including elevated S preference. Once all the S is consumed as sulfides, the abundances (up to 4 wt%) and low Fe abundances (less composition is renormalized with these components than 2.5 wt%) [3, 6]. The S and Fe abundances were removed to produce a S-free composition. We then used used to quantify Mercury’s highly reduced state, i.e., this composition to calculate the normative mineralogy between 2.6 and 7.3 log10 units below the Iron-Wüstite of each geochemical region following the steps of [18]. (IW) buffer [7, 8]. This fO2 is lower than any of the other A modification that we made to this classical calculation terrestrial planets in the inner Solar System [9–11] and was the treatment of MnO. Typically, MnO is assumed has important consequences for the thermal and to act like FeO. Given the highly reducing nature of magmatic evolution of Mercury, its surface mineralogy Mercury [7, 8], however, as well as the low amount of and geochemistry, and the petrogenesis of the planet’s Fe and Ti on the surface [3, 19], we have elected to force magmas [3, 7, 12–16]. Although MESSENGER has all Mn that is left after making MnS into a manganosite revealed substantial geochemical diversity across the component (MnO). The resulting compositions are surface of Mercury, until now, there have been only given in Table 1. limited efforts to understand the mineralogical and petrological diversity of the planet [13, 14, 16]. Here we present a systematic and comprehensive study of the potential mineralogical and petrological diversity of Mercury. Methods: We focus our study on nine regions (Figure 1) with characteristic major element compositions [6]: (i) the high-Mg region (HMR), (ii) a sub-region of the HMR with the planet’s highest Ca and S contents (HMR-CaS), (iii) the smooth plains within the Caloris basin (CB), (iv) a subset of the northern volcanic plains (NP) with relatively high Mg content (NP-HMg), (v) a subset of the NP with relatively low Mg content (NP-LMg), (vi) the Rachmaninoff basin (RB), (vii) the high-Al regions southwest and southeast of the NP (HAl), (viii) the planet’s largest pyroclastic Figure 1. Total alkali versus silica (TAS) diagram for nine distinct deposit, located northeast of the Rachmaninoff basin geochemical units on Mercury. Dotted line is at 52 wt% SiO2. Shaded region >52 wt% SiO represents boninites. Unshaded (PD), and (ix) the intermediate terrane (IT), made up of 2 region <52 wt% SiO2 represents komatiitic compositions. intercratered plains and highly-cratered terrain. We 47th Lunar and Planetary Science Conference (2016) 1476.pdf

Table 1. CIPW norm calculations (wt%) for each of the distinct geochemical regions. Detection limits for Cr, Mn, and Ti from [3] are used as upper limit values for these elements. HMR HMR-CaS CB NP-HMg NP-LMg RB HAl PD IT FeS 2.38 2.61 1.19 2.41 1.84 2.32 1.16 0.21 2.20 CrS 0.76 0.74 0.88 0.81 0.85 0.77 0.82 0.00 0.82 TiS2 1.66 1.62 0.63 0.98 1.85 1.68 1.77 0.00 1.13 MnS 0.75 0.73 0.86 0.00 0.27 0.76 0.21 0.00 0.00 MgS 0.36 0.75 0.07 0.00 0.00 0.20 0.00 0.00 0.00 CaS 0.03 0.08 0.01 0.00 0.00 0.02 0.00 0.00 0.00 Quartz 0.00 0.00 6.12 0.00 0.00 0.00 0.00 0.00 0.00 Plagioclase 40.21 32.42 57.53 46.63 57.34 41.28 57.38 45.30 50.95 Orthoclase 0.95 0.95 0.53 1.42 1.06 0.95 0.95 0.96 0.65 Nepheline 0.00 3.10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Corundum 0.00 0.00 0.28 0.00 0.00 0.00 0.25 0.00 0.00 Diopside 18.04 22.43 0.00 6.30 18.29 16.67 0.00 16.77 3.80 Hypersthene 0.28 0.00 31.91 23.51 9.26 4.04 30.02 3.59 31.49 Olivine 34.59 34.58 0.00 15.95 8.74 31.34 6.99 31.36 7.17 Ilmenite 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.18 0.00 Sphene 0.00 0.00 0.00 1.35 0.00 0.00 0.00 0.00 1.13 MnO 0.00 0.00 0.00 0.65 0.48 0.00 0.48 0.63 0.66

Results: Our resultant mineralogy (Table 1) of the Discussion: Our results indicate that Mercury’s mercurian surface includes FeS (0.21–2.68 wt%), CrS surface possesses a diverse set of rocks, with a wide (0–0.88 wt%), TiS2 (0–1.85 wt%), MnS (0–0.86 wt%), range of SiO2 content, alkali content, and major element MgS (0–3.23 wt%), CaS (0–0.33 wt%), quartz (0–7.90 compositions. The compositional diversity of these nine wt%), plagioclase (32.42–58.35 wt%), orthoclase geochemical regions likely results in a diverse surface (0.53–1.42 wt%), nepheline (0–3.10 wt%), corundum mineralogy, as indicated by CIPW norm calculations (0–0.80 wt%), hypersthene (0–37.13 wt%), diopside (0– (Table 1). The primary mineralogy of the surface rocks, 37.13 wt%), olivine (0–34.59 wt%), ilmenite (0–1.18 however, are likely dominated by forsterite, enstatite, wt%), sphene (0–1.35 wt%), and MnO (0–0.66 wt%). and albitic plagioclase, as indicated by all the Plagioclase is the dominant mineral across all compositions falling within a forsterite-enstatite-albite geochemical regions, consistent with the results of [4, triangle on a TAS diagram (Fig. 2). 14]. All compositions are hypersthene normative with The olivine-normative nature of the surface is an the exception of the HMR-CaS, which is slightly important finding because it indicates that enstatite nepheline normative (3.1%). This difference in chondrites and aubrites may not be as good as petrologic normative mineralogy could have implications for the analogs for Mercury as previously thought [3, 20]. degree of homogenization of the mantle, as well as the These meteorites are dominated by enstatite (with only petrogenetic processes that produced the geochemically minor olivine), whereas the Mercury’s surface diverse surface. By including the detection limits of Cr, compositions, partly because of their Na-rich nature, are Mn, and Ti into the oxide composition, minor CrS, highly forsterite normative. Phase equilibrium studies MnS, ilmenite, sphene, and MnO are produced, which also indicate olivine-rich mantle sources on Mercury. would not otherwise be present. The abundances of References: [1] Solomon S. C. et al. (2001) PSS, 49, 1445– these components should be considered maximum 1465. [2] Evans L. G. et al. (2012) JGR Planets, 117, E00L07. possible values. [3] Nittler L. R. et al. (2011) Science, 333, 1847–1850. [4] Peplowski P. N. et al. (2014) Icarus, 228, 86–95. [5] Weider S. Z. et al. (2012) JGR Planets, 117, E00L05. [6] Weider S. Z. et al. (2015) EPSL, 416, 109–120. [7] McCubbin F. M. et al. (2012) GRL, 39, L09202. [8] Zolotov M. Y. et al. (2013) JGR Planets, 118, 138–146. [9] Herd C. D. K. (2008) Rev. Mineral. Geochem., 68, 527–553. [10] Sharp Z. D. et al. (2013) EPSL, 380, 88–97. [11] Wadhwa M. (2008) Rev. Mineral. Geochem., 68, 493–510. [12] Brown S. M. and Elkins-Tanton L. T. (2009) EPSL, 286, 446–455. [13] Charlier B. et al. (2013) EPSL, 363, 50–60. [14] Stockstill-Cahill K. R. et al. (2012) JGR Planets, 117, E00L15. [15] Vander Kaaden K. E. and McCubbin F. M. (2015) JGR Planets, 120, 195–209. [16] Vander Kaaden K. E. and McCubbin F. M. (2016) GCA, 173, 246–263. [17] Vander Kaaden K. E. et al. (2015) LPS, 46, Abstract #1364. [18] Johannsen A. (1931) A descriptive petrography of the igneous rocks in Figure 2. TAS diagram containing end member minerals forsterite University of Chicago Press. (1931), vol. 1, pp. 88–92. (Fo), enstatite (En), and Albite (Ab). Fields for Mercury’s [19] Weider S. Z. et al. (2014) Icarus, 235, 170–186. [20] Burbine geochemical regions are the same as in Figure 1. T. H. et al. (2002) Meteorit. Planet. Sci., 37, 1233–1244.